Meteorology I Exam 2 Chapter 4

Explain two ways the relative humidity may be changed.

(1) By changing the air's water vapor content, as water vapor is added to the air, with no change in air temp, the RH increases, and, as water vapor is removed from the air, the RH decreases (2) By changing the air temperature, with no change in water vapor content, an increase in air temp lowers the RH, while a decrease in air temp raises the RH

1. What is the 'wet-bulb temperature'? 2. What is the 'wet-bulb depression'? 3. On what instrument for measuring atmospheric moisture can you find a wet and dry bulb?

1. This is the lowest temperature that can be reached by evaporating water into the air. 2. The temperature difference between the wet bulb and dry bulb (air temperature) is called the wet-bulb depression. 3. On an aspirated or sling psychrometer.

In a volume of air, how does the 'actual vapor pressure' differ from the 'saturation vapor pressure'?

Actual vapor pressure: air's total or current water vapor content Saturation vapor pressure: how much water vapor is necessary to make the air saturated at any given temperature or the maximum amount of water vapor the air can hold at a given temperature. Equal when RH=100%

The following is an example that pertains to the pervious question.

An air temperature of 10˚F with an actual vapor pressure of 1.9 mb and saturated vapor pressure of 2.4 mb would have a RH of 79%. An air temperature of 50˚F with an actual vapor pressure of 4.6 mb and saturated vapor pressure of 12.3 mb would have a RH of 37%. The 50˚F air mass actual contains more moisture than the 10˚F air mass (4.6 mb vs 1.9 mb) but has a lower RH (37% vs 79%)

What does saturation vapor pressure primarily depend upon?

At higher air temperatures, it takes more water vapor to saturate the air. More vapor molecules will exert a greater pressure. Therefore, saturation vapor pressure depends primarily on the air temperature.

Explain why the air on a hot, humid day is less dense than on a hot, dry day.

At the same temperature and at the same atmospheric level (pressure), humid air weighs less than dry air. This is due to the fact that water vapor molecules weigh less than the dry air molecules that they are replacing. (The molecular weight of water vapor = 18, while the molecular weight of dry air = 29)

Why do hot, humid summer days usually feel hotter than hot, dry summer days?

Because the evporation rate is slower

How does condensation differ from precipitation?

Condensation: small cloud droplets that are visible, but too small to fall out of the sky. Precipitation: cloud particles, liquid or solid, large enough to fall to the earth's surface.

Basically, how do the three states of water differ?

Gas, water vapor molecules move about quite freely, mixing well with neighboring atoms and molecules Liquid, the water molecules are closer together (greater density than ice), and so they constantly jostle and bump each other. Solid state, the molecules arrange themselves into an orderly pattern, with each molecule more or less locked into a rigid position, able to vibrate, but not able to move about freely.

Explain why, during a summer day, the relative humidity will change.

In many places, the air's total vapor (moisture) content varies only slightly during an entire day, and so it is the changing air temperature that primarily regulates the daily variation in RH. This would cause the highest RH values to occur during the coldest part of the day, early morning, and the lowest RH values to occur during the warmest part of the day, mid to late afternoon. From this we can see that RH does not tell us how much water vapor is actually in the air, rather, it tells us how close the air is to being saturated at a given temperature.

What are the primary factors that influence evaporation?

Occurs when more water molecules are leaving a water surface than returning: higher water temperature, higher wind speed, drier air temperature

RH Formula

RH = actual vapor pressure (e) divided by saturated vapor pressure (es) x 100

When the 'relative humidity' is given, why is it also important to know the air temperature?

Since relative humidity (RH) values indicate the amount of water vapor in an air mass relative to its temperature, and that warmer air has the capacity to hold more water vapor, a high RH value in a cold air mass could actually contain less actual water vapor than a low RH value in a warm air mass.

How is the difference between dew point and air temperature (dew point depression) related to the relative humidity?

Since the dew point is a direct measure of actual water vapor in the air, the difference between air temperature and dew point can indicate whether the relative humidity is low or high. When the two are far apart, the relative humidity is low, when the two are close together, the relative humidity is high.

What does the 'relative humidity' represent?

The relative humidity (RH) is the ratio of the amount of water vapor actual in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure). Or in other words, it is the ratio of the air's water vapor current content to its possible full capacity.

Explain why it takes longer to cook vegetables in the mountains than at sea level.

The saturation vapor pressure is directly related to the temperature of the liquid, higher water temps produce higher vapor pressures. Hence, any change in atmospheric pressure will change the temp at which water boils. An increase in air pressure raises the boiling point, while a decrease in air pressure lowers it. Since pressure decreases with height, the higher you go, the lower the boiling point, and the longer it will take to cook vegetables.

What is the 'dew-point temperature'?

This is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. Addition of water vapor to the air increases the dew-point temperature; removing water vapor lowers the dew-point temperature.

Explain why condensation occurs primarily when the air is cooled.

When the air is chilled, the molecules move more slowly and are more apt to stick and condense to the nuclei (microscopic bits of dust, smoke, salt, and other solid particles)